1. Field of the Invention
The present invention relates to a method of forming a metal-semiconductor layered structure provided on an electrically insulating substrate, and also relates to a light emitting semiconductor device including such a layered structure.
2. Description of the Related Art
Heretofore, there has been earnestly required to develop a practically usable blue light emitting semiconductor laser formed by a basic material of gallium nitride (GaN). FIG. 1 is a schematic view showing a known semiconductor laser formed by a basic material of gallium nitride. It is impossible or practically difficult to obtain a single crystal wafer of gallium nitride, and therefore use is made of a sapphire substrate 1, and a thin buffer layer 2 of gallium nitride 2 is formed on the sapphire substrate 1, a first cladding layer 3 formed by a thick n-type gallium nitride layer having a thickness of about 3 .mu.m is deposited on the buffer layer 2, an active layer 4 is formed on the first cladding layer 3, a second p-type cladding layer 5 is formed on the active layer 4, and a p-electrode 6 is formed on the second cladding layer 5. A part of a corner of such an assembly is removed by a suitable patterning treatment to expose a part of the first cladding layer 3, and an n-electrode 7 is formed on the thus exposed surface of the first cladding layer.
In such a known semiconductor laser, a current path is formed from the n-electrode 7 to the p-electrode 6 via the first cladding layer 3, active layer 4 and second cladding layer. In the known semiconductor laser, since the n-electrode 7 is provided on the exposed surface of the corner portion of the first cladding layer 3, carriers introduced from the n-electrode travel laterally along the exposed surface and then arrive at the active layer 4. Therefore, the carrier travelling path becomes longer and has a relatively high electric resistance, and thus a substantial amount of electric power is consumed therein. This also causes an increase in an operating voltage.
Moreover, in order to provide the n-electrode 7, the assembly of the first and second cladding layers 3 and 5 and active layer 4 has to be partially removed by any suitable patterning process and the manufacturing process is liable to be complicated and through-put is decreased.
The above mentioned problems of the known semiconductor laser could be completely removed if a metal layer serving as an electrode is first formed on an insulating substrate and then a single crystal semiconductor layer is formed directly on the metal layer. In such a structure, carriers introduced from the electrode could be efficiently supplied to the active layer through a thin cladding layer, and therefore an electric resistance of the carrier travelling path could be materially lowered.
Not only the above mentioned semiconductor laser, but also in solar cell and photo-detector, the above structure of the substrate-metal layer-semiconductor layer has been widely used, and thus if the single crystal semiconductor layer could be formed directly on the metal layer formed on the insulating substrate, an efficiency of the device and manufacturing process could be improved to a great extent.
However, it has not been proposed any practical method of forming a semiconductor layer directly on a metal layer formed on an electrically insulating substrate.